1. Field of the Invention
This invention relates to an implantable medical device that delivers sufficient electrical energy to cardiac tissue to defibrillate or cardiovert tachyarrhythmias and thus restore normal sinus rhythm and, more particularly, to improved driving circuits for controlling discharge of high voltage capacitors providing a biphasic cardioversion wave form shock.
2. Background Art
By way of definition, in the field of automatic implantable arrhythmia control devices, the term "cardioversion" or "cardioverter" refers to the process of and device for discharging relatively high energy electrical pulses into or across cardiac tissue to arrest a life threatening tachyarrhythmia. Cardioversion pulses may or may not be synchronized with a cardiac depolarization or rhythm and may be applied to arrest a malignant ventricular tachycardia or ventricular fibrillation with a selectable or programmable pulse energy. The arrest of ventricular fibrillation by such pulses is referred to as "defibrillation" (a form of cardioversion), and "defibrillators" have been characterized as a form of cardioverter. In the following description and claims, it is to be assumed that these terms are interchangeable, and that use of one term is inclusive of the other device or operation, unless specific distinctions are drawn between them in the context of the use.
Current implantable devices for the treatment of tachyarrhythmias, e.g. the MEDTRONIC Model 7217 PCD device, provide programmable staged therapies including anti-tachycardia pacing regimens and cardioversion energy and defibrillation energy shock regimens in order to terminate the arrhythmia with the most energy efficient and least traumatic therapies (if possible), as well as single chamber bradycardia pacing therapies. The Model 7217 PCD device provides a programmable energy, single polarity wave form, shock from the discharge of a high voltage output capacitor bank through a pair of electrodes disposed in relation to the heart.
Commonly assigned U.S. Pat. No. 5,163,427 to Keimel discloses an implantable cardioverter/defibrillator system which is capable of providing three defibrillation pulse methods, with a minimum of control and switching circuitry. The output stage is provided with two separate output capacitor banks which are sequentially discharged during sequential pulse defibrillation and simultaneously discharged during single or simultaneous pulse defibrillation through a two or three electrode system.
Other cardioversion pulse wave shapes have been proposed in conjunction with a variety of electrode systems in order to achieve more efficient cardioversion, including bi-phasic or multi-phasic wave form shocks generated in rapid sequence and applied to the same or separate electrode systems as described in the above referenced '107 application and in U.S. Pat. Nos. 4,800,833 to Winstrom, 4,830,006 to Haluska et al, 4,953,551 to Mehra, 5,178,140 to Ibrahim, and 4,850,357 to Bach. Despite the additional complexity, it is expected that cardioversion may be achieved more rapidly after the onset of an arrhythmia and at lower current consumption. In order to achieve low current consumption, these stimulation therapy regimens require rapid and efficient charging of high voltage output capacitors from low voltage battery power sources as well as efficient sequential (or simultaneous) discharge of the capacitors through the electrode systems employed.
Generally speaking, it is necessary to employ a DC-DC converter to convert electrical energy from a low voltage, low current power supply to a high voltage energy level stored in a high energy storage capacitor. A typical form of DC-DC converter is commonly referred to as a "flyback" converter which employs a transformer having a primary winding in series with the primary power supply and a secondary winding in series with the high energy discharge capacitors. An interrupting circuit or switch is placed in series with the primary coil and battery. Charging of the high energy capacitors is accomplished by inducing a voltage in the primary winding of the transformer creating a magnetic field in the secondary winding. When the current in the primary winding is interrupted, the collapsing field develops a current in the secondary winding which is applied to the high energy capacitors to charge them. The repeated interruption of the supply current charges the high energy capacitors to a desired level over time. Such DC-DC converters are disclosed in wherein charging circuits are disclosed which employ flyback oscillator voltage converters which step up the power source voltage and apply charging current to output capacitors until the voltage on the capacitors reaches the programmed shock energy level.
In sequential pulse, multi-phasic systems, two or more output capacitors are charged and discharged through separate discharge circuits arranged in a bridge circuit configuration so that the sequentially generated shocks applied to the same electrode pathway(s) have opposite polarity. The discharge of the high voltage capacitors is typically effected by connecting the charged capacitors to the electrodes in discharge circuit paths through high voltage, high current conducting, Insulated Gate Transistors (IGTs) or metal oxide semiconductor field effect transistors (MOSFETs or power FETs), either employed alone or in electrical series with high voltage thyristers or "triacs". In the above referenced '107 application and '006, and '427 patents, IGTs or power FETs are switched into conduction by dedicated drive circuits which respond to low voltage control signals.
These low impedance, high current conducting switches are necessary to make and break the series electrical connection of the high voltage capacitors with the electrode/heart tissue load. The function of these switches must be tightly controlled to assure proper timing of the sequentially generated mono-phasic or bi-phasic shock impulses and to prevent destruction of the high voltage output circuit by the unintentional insertion of the switches directly across the high voltage capacitors. Noisy switch operation must also be suppressed.
In order to electrically isolate the high voltage discharge circuits from the low voltage control circuits and microprocessor based control system, isolation transformers or optical isolators (opto-couplers) or capacitive coupling and common mode rejection circuits have been proposed. In the '006, '357 and '531 patents, transformers are employed to couple discharge control signals to drive circuits. As stated in the '140 patent, such transformers are bulky, and the transformer cores are susceptible to external magnetic fields.
The optical isolators and driver circuits employed in the '427 patent do not suffer from these drawbacks, but the miniaturization of the opto-coupler components does result in large isolation capacitances. These capacitances can introduce switching transients when the discharge current is abruptly switched on and off, which could result in re-triggering through the driver circuit.